1. Field of the Invention
The present invention relates to a process for producing protective coverings for lamps, polymethyl (meth)acrylate being given light-scattering or diffusing properties. This is achieved via admixture of scattering particles to the plastics matrix, the refractive index of the scattering particles differing from that of the polymethyl(meth)acrylate.
2. Discussion of the Background
Mixtures of this type composed of polymethyl (meth)acrylate and scattering particles are especially suitable for the lighting sector. For example, EP-A 0 656 548 describes uniformly illuminated optical conductor plates composed of a methyl methacrylate matrix and of scattering particles embedded therein, composed of a crosslinked polymer which may contain acrylic esters, methacrylic esters and styrene. The order of size of the scattering particles is from 2 to 20 μm, and they are incompatible with the plastic of the matrix.
There are also known lampshades which comprise plastics particles as scattering media. For example, the document JP11179856 describes multilayer sheets with at least one layer which encompasses a polymethyl methacrylate matrix and also encompasses crosslinked polymethyl methacrylate beads as a scattering/matting agent, the content of the beads being in the range from 0.5 to 25% by weight. The size of the beads is in the range from 3 to 30 μm, and the examples here describe only sheets of thickness 2 mm, which comprise about 3% by weight of scattering beads whose size is about 6 μm.
The Japanese specification JP-A 418346 describes light-scattering multilayer sheets with a layer composed of methyl methacrylate, this layer comprising from 1 to 20% by weight of crosslinked styrene particles whose average size is from 1 to 30 μm.
The moulding compositions described are usually further processed via extrusion to give screens or optical conductor plates. For the production of complex moulded parts, e.g. specialized headlamp covers, and parts with cavities or perforations, the extruded products have to be further processed via forming processes. The high costs which arise via the additional operation are a disadvantage of this procedure.